Such a process is for example known from patent application EP1574539A1. This document describes a process wherein PTA and EG are reacted in the presence of a catalyst system consisting of 15-150 ppm of Sb-compound and 40-160 ppm of Zn-compound as active catalyst components and 10-30 ppm of phosphoric acid as stabilizing component. In the reported experiments catalyst components were added together with PTA, EG, and optionally other components at the beginning of esterification step a), while the phosphoric acid was added at the end of esterification. Compared with a standard Sb-catalyst, this catalyst system is indicated not only to increase productivity in both melt-phase polycondensation and in a subsequent solid-phase polycondensation (SSP) step, but also to enhance optical properties of the PET; i.e. improved clarity and reduced greyish colour generally ascribed to Sb-catalyst residues.
Polyesters like PET are well-known in the art, and are widely applied in applications like textile and industrial fibres, films and sheets, and containers, especially bottles. Initial PET production employed dimethyl terephthalate (DMT) and ethylene glycol (also called monoethylene) glycol (EG) as precursors, but most production plants currently use purified terephthalic acid (PTA) and EG as raw materials, because of process economic reasons. In this case, first an oligomer or low molecular mass prepolymer is formed by esterification of PTA with a molar excess of EG to form diethyleneglycol terephthalate (also called bis hydroxyethyl terephthalate) and oligomers thereof (DGT), with water being the main by-product distilled off (step a). This step is generally self-catalysed, but may be accelerated by adding catalyst. DGT is further subjected to polycondensation by transesterification to form higher molecular mass polyester (step b). In this step, DGT is heated to about 280° C. under high vacuum to carry out the melt-phase polycondensation reaction with removal of EG liberated in the polycondensation reaction. Because the transesterification is a slow reaction, the polycondensation step is generally catalysed. This catalyst can be added in step b), but it can also already be included in step a). The melt is discharged and made into pellets after it reaches a desired molecular mass, reflected by intrinsic viscosity (IV) values. Commercial-scale PET production is generally based on a continuous PTA system employing several reactors in series, as described for example by S. M. Aharoni in “Handbook of Thermoplastic Polyesters”, vol. 1, chapter 2, Editor S. Fakirov, Wiley-VCH, 2002; and by V. B. Gupta and Z. Bashir in “Handbook of Thermoplastic Polyesters”, vol. 1, chapter 7, Editor S. Fakirov, Wiley-VCH, 2002. Typically, such a system uses a vessel in which EG, PTA, catalyst and additives are mixed to form a paste; one or more esterification reactors; one or more pre-polycondensation reactors, followed by a high-vacuum, finisher reactor for the final stages of polycondensation. The polyester formed may be extruded into strands, quenched under water and cut to form pellets or chips. PET used in film and fibre applications typically has an IV in the range of 0.55 to 0.65 dL/g; PET films and fibres can also be produced directly by extruding the melt from the poycondensation reactor. For PET bottle grade resin, polymers with IV in the range of 0.75 to 0.85 dL/g, and having low residual acetaldehyde are generally required. In this case, a split process is used to attain this IV value while attaining a low amount of acetaldehyde. The general practice is to make polymer chips with an intermediate IV of about 0.63 dL/g by melt-polycondensation, and then increase the IV by subsequent solid-state polycondensation (SSP). This split procedure allows production of a high IV resin with minimal quantities of acetaldehyde, which is a degradation by-product that affects the taste of beverages packed in PET bottles. Diethylene glycol (DEG) is a diol generated from ethylene glycol via a side-reaction and is also incorporated in the PET chain. Presence of DEG as comonomer reduces the glass transition and melting temperature of the PET, but too high levels are undesirable. The melt-phase and SSP technology is described for example in Encyclopaedia of Polymer Science and Engineering, 2nd ed, volume 12, John Wiley and Sons, New York (1988), and in “Handbook of Thermoplastic Polyesters”, vol. 1, chapter 7, Editor S. Fakirov, Wiley-VCH, 2002.
The catalysts currently employed in more than 90% of industrial PET production are based on antimony (Sb), mostly on antimony triacetate or antimony trioxide. Typically, about 200-300 ppm Sb (based on PET) is used to provide sufficiently fast reaction. A disadvantage of using antimony-based catalyst compounds is the greyish colour of PET that is reported to result from precipitation of antimony metal particles. Moreover, antimony is rather expensive and shows some environmental concerns. Various publications addressed catalysts systems for PET that combine Sb with a second or third metal compound to result in some synergistic effect. For example, U.S. Pat. No. 5,008,230 and U.S. Pat. No. 5,166,311 describe a tri-component catalyst based on antimony, 5-60 ppm of cobalt and/or zinc, and 10-150 ppm of zinc, magnesium, manganese or calcium. As zinc can be present as the second and third component, a bi-metallic composition with 150-650 ppm of antimony and 5-210 ppm of zinc is included. The catalyst components can be added at any time before or during polycondendation, and would allow reducing melt-polycondensation times by at least one-third, compared to the conventional antimony catalyst. Other patent publications covering Sb—Zn catalyst compositions include U.S. Pat. No. 5,162,488, and EP0399742. U.S. Pat. No. 5,623,047 claims that the optical appearance of the PET made from the PTA process can be improved by introducing alkali metal acetate as third component besides antimony and at least one of cobalt, magnesium, zinc, manganese and lead. U.S. Pat. No. 5,608,032 discloses a catalyst system that contains 10-1000 ppm Sb, 10-500 ppm of at least one of Co, Mg, Zn, Mn, Ca and Pb, and 10-500 ppm of a P-compound; all components being added to the esterification step.
In a PET production process as described above, EG formed is removed from the reaction mixture during the polycondensation step, such that the equilibrium reaction will proceed. The EG removed, also referred to as spent glycol, is preferably re-used or recycled in the process for efficiency and cost control reasons. In DE19537930 a continuous process for making polyesters like PET is described, wherein the EG distilled off is used—after removal of water and other low boiling point components—in the initial paste making step in combination with fresh or virgin EG and phosphoric acid (or a glycol ester thereof). In this process, the metal catalyst—based on Sb, Ti, Ge, Sn, Zn, Mn or mixtures thereof—is only to be added after 65-80% of the total esterification time has elapsed to control optical properties of the PET. U.S. Pat. No. 4,146,729 relates to a continuous direct esterification process for PET, wherein removed EG is recovered for re-use in the slurry making step by applying a rectification column. In WO9916537 the use of cross-flow membrane filtration is proposed as alternative to distillation steps in order to purify spent glycol and make it suited for recycling to paste making. These documents are generally silent on effects of EG re-use on the PET polymerisation process or polymer properties. In WO2004076513 it is taught that improved colour and clarity of PET results if spent EG is purified by hydrogenating impurities in the glycol before re-using it in the process.
A disadvantage of the known process for making PET from EG, PTA and optionally up to 30 mol % comonomer, using a catalyst system essentially consisting of Sb-, Zn- and P-compounds, comprising the steps of a) esterifying EG and PTA to form diethyleneglycol terephthalate and oligomers (DGT), and b) melt-phase polycondensing DGT to form polyester and EG as described in EP1574539, is that in case spent EG is recycled to the esterification step a)—instead of using only virgin EG—the PET that is obtained shows pronounced haze in moulded articles like bottles.